Recently, we have developed nanocrystals capable of delivering anticancer drugs. These nanocrystals with possess low toxicity, high drug loading ratio, and overcome multidrug resistance (MDR) in cancer cells in vitro and in vivo. The overall goal of this research proposal is to further develop and investigate multifunctional nanocrystals for MDR cancer therapy. The scientific basis for this proposal originated from our three-phase nanoparticle engineering (3PNE) method recently developed, which includes phase 1, amorphous precipitate; phase 2, hydrated amorphous aggregate; and phase 3, stabilized nanocrystals (NCs). The 3PNE has been applied to the development of rod-shaped NCs for effective anticancer drug delivery. To achieve this goal, we propose three aims: specific aim 1, to examine the mechanisms of the NCs for their ability to enhance physical stability, overcome MDR, and their in vivo behavior; specific aim 2, to develop NCs that carry target ligands to deliver PTX to tumor; and specific aim 3, to establish the NCs for co-delivering a hydrophobic drug (i.e., Paclitaxel) and a hydrophilic anti-cancer drug (i.e., 5-fluorouracil) to synergistically kill tumor cells through different cellular mechanisms. Accomplishing these specific aims will further develop efficient nano-drug carriers for the treatment of cancer, particularly MDR cancer.